The present invention relates to a process for operating a gas phase reactor, preferably fluidized bed polymerization reactor in the presence of a Ziegler-Natta catalyst to reduce resin stickiness and hexane extractables and improve physical properties.
In the gas phases polymerization of polyethylene, and particularly fluidized bed polymerization it is desirable to produce a free flowing granular polymer. If the polymer becomes sticky due to a number of factors such as production of oligomers and/or adsorption of comonomer on the polymer the particles tend to agglomerate. If particles start to agglomerate a number of problems may arise. It may be difficult to continue to keep the particles in a fluidized state. The pressure drop across a fluidized bed of polymer particles should be such that it is slightly greater than the mass of the bed divided by the cross section area of the bed. Typically in a fluidized bed gas phase reactor the flow rate of gas through the bed is from about 1.5 to 10, preferably 2 to 6, most preferably from 3 to 5 times the minimum flow rate to fluidize the bed. The superficial gas velocity is typically 0.2 to 0.5 ft/sec above the minimum velocity to fluidize the bed. Typically the superficial gas velocity is from 0.7 ft/sec (0.214 m/sec) to 5.0 ft/sec (1.5 m/sec), preferably from 1 ft/sec (0.305 m/sec) to 3.5 ft/sec (1.07 m/sec). However, the superficial gas velocity is related to the weight average particle diameter, and the density of the gas. If the particles are xe2x80x9cstickyxe2x80x9d and tend to agglomerate then the superficial gas velocity must increase to maintain that larger particle in a fluidized state. Additionally the flow of gas through the fluidized bed helps to remove the heat of polymerization. Further xe2x80x9cstickyxe2x80x9d polymer particles are difficult to recover from the reactor, as they tend to plug transfer lines and may also agglomerate in the degassing apparatus, which is used to remove unreacted monomer and comonomer.
U.S. Pat. No. Re 33,683, issued Mar. 22, 1988, reissued Sep. 3, 1991, assigned to Mobil Oil Corporation teaches that if a conventional Ziegler-Natta catalyst is activated only with trimethyl aluminum (TMA) in an amount from 15 to 300, preferably 30 to 150, most preferably from about 40 to 80 ppm in the resulting polymer, the resulting polymer has reduced hexane extractables. The reference teaches the co-catalyst may be used in an amount to provide from 6 to 80, preferably from 8 to 30 moles of co-catalyst (i.e. moles of aluminum) per mole of Ti. The present patent application has been restricted to exclude trimethyl aluminum as an activator.
WO 01/05845 (PCT/US00/19138) published Jan. 25, 2001 in the name of Union Carbide Corporation teaches that another activator such as triethyl aluminum (TEAL) may be used in the activation of the Ziegler-Natta catalysts. However, the patent teaches the molar ratio of total Al:Ti is from 1:1 to 15:1. This is much lower than the ratio of aluminum to titanium according to the present invention.
Canadian Patent Application 2,193,758 laid open Jul. 5, 1997 contains similar teaching to those in WO 01/05845 except that the total atomic (molar) ratio of Al:Ti is from 10:1 to 22:1. However, the aluminum co-catalyst is limited to triethyl aluminum. The present invention is limited to a ratio of total Al (i.e. aluminum in the catalyst and the co-catalyst) to titanium (from the catalyst) of not less than 25:1, typically from 25:1 to 80:1.
The present invention seeks to provide a novel method to operate a gas phase polymerization reactor so that the hexane extractables are lower and in preferred embodiments, with higher alkyl olefin comonomers films of the resulting resin may have a higher dart impact strength.
The present invention provides in a process for the gas phase polymerization of ethylene and from 0 to 20 weight % of one or more C4-8 copolymerizable alpha olefin monomers in the presence of a supported Ziegler-Natta catalyst co-catalyzed with tri C2-6 alkyl aluminum, the improvement of controlling the feed of said tri C2-6 alkyl aluminum co-catalyst to the reactor to provide from 10 to 50 ppm of aluminum from the co-catalyst based on the polymer production rate provided that the molar ratio of total Al from the catalyst and co-catalyst:Ti from the catalyst is not less than 25:1 (typically from 25:1 to 80:1).
The present invention also provides a process to control a gas phase polymerization of ethylene and from 0 to 20 weight % of one or more C4-8 copolymerizable alpha olefin monomers in the presence of a supported Ziegler-Natta catalyst co-catalyzed with tri C2-6 alkyl aluminum, comprising maintaining the molar ratio of total Al from the catalyst and co-catalyst:Ti from the catalyst from 25:1 to 80:1 and controlling the feed of said tri C2-6 alkyl aluminum co-catalyst to the reactor to provide from 10 to 50 ppm of aluminum from the co-catalyst based on the polymer production rate.
In a particularly preferred embodiment the present invention provides a process for the gas phase polymerization of ethylene and one or more C3-8 copolymerizable alpha olefin monomers in the presence of a supported Ziegler-Natta catalyst comprising an aluminum compound of the formula Al((O)aR1)bX3xe2x88x92b wherein a is either 0 or 1, b is an integer from 1 to 3, R1 is a C1-10 alkyl radical and X is a chlorine atom, a titanium compound of the formula Ti(OR2)cXd wherein R2 is selected from the group consisting of a C1-4 alkyl radical, a C6-10 aromatic radical, and a radical of the formula xe2x80x94COR3 wherein R3 is selected from the group consisting of a C1-4 alkyl radical and a C6-10 aromatic radical, X is selected from the group consisting of a chlorine atom and a bromine atom, c is 0 or an integer up to 4 and d is an integer up to 4 and the sum of c+d is the valence of the Ti atom; a magnesium compound of the formula (R5)eMg X2xe2x88x92e wherein each R5 is independently selected from the group consisting of C1-4 alkyl radicals and e is 0, 1 or 2, a C1-6 alkyl halide and optionally an electron donor, a molar ratio of Al:Ti from 1:1 to 15:1; a molar ratio of Mg:Ti from 1:1 to 20:1; a molar ratio of halide from the alkyl halide to Mg from 1:1 to 8:1; and a molar ratio of electron donor to Ti from 0:1 to 15:1; said catalyst being co-catalyzed with tri C2-6 aluminum, the improvement of controlling the molar ratio of total Al from the catalyst and co-catalyst:Ti from the catalyst from 25:1 to 80:1 and the feed of said tri C2-6 alkyl aluminum from the co-catalyst to the reactor to provide from 10 to 50 ppm of aluminum (Al ppm) based on the polymer production rate.